x86: increase default idle stack size
x86: increase default idle stack size

With MMU features enabled, we are using 248 out of 256
available bytes on 32-bit. This is extremely uncomfortable, relax
to a larger value like several other arches.

Signed-off-by: Andrew Boie <aab045b28152628be6af0e556050d0381fd0a34d@intel.com>

galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr